The present invention relates to implantable sensors, and more particularly to monitoring such sensors for proper performance. Even more particularly, the present invention relates to integrity tests that are performed on a regular basis in order to confirm proper sensor operation. A preferred sensor with which the present invention may be used is an implantable glucose sensor.
Implantable sensors are sensors adapted to be implanted within living tissue, e.g., within a living patient. The patient may comprise an animal or a human. Such implantable sensors are typically used to monitor one or more physiological parameters associated with the patient. For example, an implantable sensor may monitor a patient""s blood or other body fluids for the presence or absence of a specific substance. Other implantable sensors may monitor the patient""s body temperature. While a preferred sensor for use with the present invention comprises an implantable glucose sensor, or groups of glucose sensors, it is to be understood that the invention may be used with any type of implantable sensor(s). It is also to be understood that the principles underlying operation of an implantable sensor apply equally well to any sensor that is to remain unattended and submerged or immersed within a hostile environment, e.g., within a saline solution such as seawater, for a prolonged period of time. Thus, although the sensors described herein find particular applicability to sensors adapted to be implanted within living tissue, and the description is directed to such implantable sensors, the invention may also be applied to remote sensors of any kind that must be immersed unattended in a hostile environment for long periods of time.
In general, an implantable sensor may be used to provide valuable data that assists in diagnosing or treating an illness, or to help maintain or sustain a given level of physiological, chemical, or other activity or inactivity. In the case of glucose sensors, which are typically used with some type of insulin-delivery system in order to treat diabetics, the glucose sensors provide invaluable data needed to maintain the concentration of glucose within the patient at an acceptable level. Such glucose senors must perform properly; otherwise, false data may be provided. Such false data (if acted upon) could result in the administration of an inappropriate amount of insulin, leading to death or serious injury. There is thus a critical need in the art for a sensor which is reliable and which can be monitored for proper function on a regular basis. Likewise, there is a need for a glucose sensor which must work properly within certain specific limits of accuracy.
The above and other needs are addressed by the present invention which comprises an implantable sensor that includes integral means for automatically performing a series of integrity tests that test the sensor for proper performance, and which thereby ensure that the sensor is correctly and accurately performing its intended monitoring function(s).
In accordance with one embodiment of the invention, an electrochemical sensor is provided that has a hermetically sealed portion and a non-hermetically sealed portion. The hermetically sealed portion contains electronic circuitry; and the non-hermetically sealed portion has at least one electrode associated therewith. The electronic circuitry includes means for measuring a specified parameter within body fluids or tissue to which the at least one electrode is exposed, and means for performing at least one integrity test to verify proper operation of the sensor. Preferably, the means for performing the integrity test comprises means for automatically performing the integrity test upon occurrence of a specified event, such as the passage of time in accordance with a prescribed schedule (e.g., once every hour, or once every day), or the sensing of a parameter that is out of tolerance.
A preferred embodiment of the invention comprises an implantable glucose sensor. Such glucose sensor includes, inter alia, electronic circuitry for automatically performing on a periodic basis, e.g., every 1 to 24 hours, specified integrity tests which verify the proper operation of the glucose sensor. The basic structure and operating characteristics of a preferred implantable glucose sensor adapted for use with the present invention are described generally in U.S. Pat. No 5,497,772, incorporated herein by reference. Important features and enhancements of such sensor are further described in U.S. patent application Ser. No. 08/928,867 filed Sep. 12, 1997, now U.S. Pat. No. 5,999,848; U.S. patent application Ser. No. 08/928,868 filed Sep. 12, 1997, now U.S. Pat. No. 5,917,346; U.S. patent application Ser. No. 08/928,871 filed Sep. 12, 1997, now U.S. Pat. No. 5,999,849; U.S. patent application Ser. No. 08/954,166 filed concurrently herewith, now U.S. Pat. No. 6,119,028; and U.S. patent application Ser. No. 08/953,817 filed concurrently herewith, now U.S. Pat. No. 5,081,736; all of which are assigned to the concurrently herewith; all of which are assigned to the same assignee as the present application, and all of which patent applications are also incorporated herein by reference.
The preferred glucose sensor is adapted for insertion into the venous system of a patient where it is exposed to the patient""s blood, or into other areas of the patient where it is exposed to other tissue or fluids of the patient. Once implanted, the sensor produces electrical signals, i.e., an electrical current, that is related to the sensed glucose concentration.
Most implantable sensors have one or more electrodes adapted to contact tile body tissue or fluids within which the sensor is implanted. It is through such electrodes that the sensor is able to sense the particular parameter it is designed to sense.
For example, as described in the above-referenced patent and patent applications, the preferred glucose sensor includes several electrodes, e.g., two working electrodes (W1 and W2), at least one reference electrode (REF1), and a counter electrode (CNTR). Some of the electrodes, e.g., the working electrodes and the counter electrode, are made or coated from platinum, while the reference electrode is typically made from or coated with silver chloride. Some of the electrodes are surrounded by a prescribed enzyme, typically in the form of a gel. A preferred enzyme used for this purpose is glucose oxidase (referred to herein as xe2x80x9cGOXxe2x80x9d). All of the electrodes are further submersed in a suitable conductive fluid, e.g., a saline solution.
The electrodes of the preferred glucose sensor are typically formed on one side of a substrate, with membranes being formed to confine the conductive fluid and/or GOX in the areas needed to expose the electrodes. On the other side of the substrate, electronic circuitry is formed that connects the electrodes appropriately so that the desired electrochemical activity can be monitored and used as a measure of the concentration of glucose to which the sensor is exposed. Such circuitry includes not only circuits that monitor the sensed glucose concentration (which is done, as explained below, by monitoring the current flow between the electrodes, which provides a measure of the oxygen concentration, which oxygen concentration in the presence of an enzyme is inversely proportional to the concentration of glucose), but also includes data processing circuitry to preliminary process the sensed data (e.g., the measured current) and transmit it over a two-line connection cable with a controller circuit to which the sensor is connected. The circuitry is hermetically sealed, and non-exposed portions of the electrodes are similarly sealed under a coating of aluminum oxide or alumina or other suitable insulator. Portions of the sensor are also insulated in epoxy. In a preferred embodiment, several, e.g., three, such sensors may be daisy-chained together, each operating independently of the others, yet each being in close proximity with the others so that measured data from different ones of the sensors can be compared.
In order for the sensor to perform its intended function, it is important that the electrodes and circuitry all operate as designed, and that the various insulative material or coatings used with the sensor, e.g., alumina, zirconia, wax and/or epoxy, provide the needed insulation and/or sealing properties.
In accordance with one aspect of the invention, special test circuitry is provided as part of the sensor circuitry to periodically check the integrity of the critical sensor functions and/or parameters. When necessary or desired, the results of the integrity tests are then reported by generating appropriate data signals that provide an indication of the results of such integrity tests, e.g., that warn when a given test has failed, and/or that provide test data from which a quantitative measure of the test results can be obtained.
In a preferred configuration, a plurality of sensors, e.g., three sensors, are daisy-chained together and implanted within a patient in the same general area, i.e., in the same tissue or body fluids. Each sensor operates independently of the others. If all the sensors are functioning properly, then the output data obtained from each sensor should be approximately the same. The data sensed by each sensor may thus be used as a cross-check against the data sensed by the other sensors. In a similar manner, the information obtained from the periodic integrity tests may be regularly compared and checked with the corresponding integrity test information obtained from the other sensors of the same group of chained-together sensors. In this manner, the overall integrity of the integrity tests is itself checked periodically.
It is thus an object of the invention to provide, within an implantable sensor, e.g., of the general type disclosed in U.S. Pat. No. 5,497,772, or a similar implantable electrochemical sensor, a means for automatically verifying the integrity of the sensor on a periodic basis.
It is a feature of the invention to provide electrical/electronic circuits included as part of the sensor circuitry that carry out a series of integrity tests on the sensor on a scheduled basis, and that report the results of such tests by way of test data that can be monitored over time and/or compared with similar test data obtained from other implanted sensors.